Polymer properties are dictated by a combination of monomer structure, chain length and processing. Monomer structure can often determine how the polymer coils, crystallizes, forms electrostatic or hydrogen bonds and of course dissolves. If the monomer unit provides extended conjugation along the polymer backbone, the polymer may offer conducting, semiconducting, emissive or light absorptive properties of use in organic electronic and photonic applications. Rigid monomers lead to polymers with excellent mechanical properties and/or liquid crystallinity. Finally monomer structure can also dictate miscibility with other polymers.
Chain length will normally dictate the glass temperature (Tg), diffusion rates, viscosities, CTEs, extents of mechanical crosslinking and for short chains, the melting temperature. Processing provides control of chain-chain interactions on a molecular scale as a means of controlling global properties through control of molecular alignment providing for example, toughness, transparency, conductivity etc. It is well recognized that specific polymer properties arise from specific types of monomers, degrees of polymerization and processing. Simply said, “One size does not fit all.” There are certain types of polymer (oligomer) systems that may offer much more tailorability than others such that “One size fits many.” One such system encompasses the family of compounds called silsesquioxanes (SQ) as illustrated in FIG. 1. Because of the breadth of their properties, silsesquioxanes are of considerable interest to both the academic and industrial communities, having been the subject of numerous reviews in the last 25 years.
To illustrate, random structured silsesquioxanes are often called T resins and offer a number of useful properties centered about their excellent adhesion and high temperature stability. In one form, with R as H or CH3, they are used as interlayer dielectrics processed either by spin-on or vapor deposition methods. They are also called organic silicates. In other forms they are used to form molds, as clear coats for a wide variety of substrates and are a major component of silicone based caulks for example.
Polyhedral silsesquioxanes, such as the T8 octasilsesquioxane “cubes” [RSiO1.5]8 (FIG. 1), represent a versatile class of highly symmetrical three-dimensional organosilicon compounds with well-defined nanometer size structures. The combination of a rigid silica core and a more flexible, modifiable organic shell make these compounds useful as platforms for assembling hybrid nanocomposite materials with properties intermediate between the properties of ceramics and organics.
Silsesquioxanes have been used in recent years to: (1) model catalytic surfaces, (2) generate new catalysts and (3) novel porous media, and serve as (4) NMR standards, and (5) encapsulants. The decameric T10 and dodecameric T12 cages are frequently formed alongside the T8 cube, albeit in lower yields, and their derivatives often exhibit chemical, thermal, and mechanical properties parallel to those of T8 derivatives.
Cubic or T8 silsesquioxanes are typically prepared via acid or base-catalyzed hydrolytic condensation of trifunctional organosilanes or by chemical transformation of the pendant groups on pre-existing cages. Since their initial discovery in 1946, there have been numerous studies on the synthesis of polyhedral silsesquioxanes. However, no universal preparative procedures have been established.
The R groups are selected from wide variety of aliphatic and aromatic functional groups, offering considerable potential to control the properties of any oligomeric, polymeric, and/or organic/inorganic hybrid nanocomposites that could be made from them.
In general, most of the work reported for SQs centers on monofunctional materials with seven inert R groups (typically R=iBu, cyclopentyl, cyclohexyl and occasionally phenyl) and one reactive group. There are only a few examples of difunctional octameric or heptameric partial or whole cages and even fewer examples of polymers (oligomers) that contain the SQ as part of the main chain. The reason is that isolation of these types of compounds is very difficult and there are only a few reports on their use in the production of linear polymers.